serial: serial_msm: Ensure BAM/single character mode are disabled

serial: serial_msm: Ensure BAM/single character mode are disabled

At the moment, the U-Boot serial_msm driver does not initialize the
UART_DM_DMEN register with the required value. Usually this does not
cause any problems, because there is Qualcomm's LK bootloader running
before U-Boot which initializes the register with the correct value.

It's important that this register is initialized correctly, because
the U-Boot driver does not make use of the BAM/DMA or single character
mode functionality of the UART controller. A different bootloader
before U-Boot might initialize the register differently.

For example, on DragonBoard 410c U-Boot can also be installed to the
"aboot" partition (replacing LK entirely). In this case U-Boot is
loaded directly by SBL, which seems to use the single-character mode
for some reason. In single character mode there is always just one
char in the FIFO, instead of the 4 characters expected by
msm_serial_fetch(). It also causes issues with "earlycon" later in
the Linux kernel, which tries to output 4 chars at once,
but only the first char will be written.

This causes early UART log in Linux to be corrupted like this:

    [ 00ano:ameoi .Q1B[ 00ac _idaM00080oo'ahani-lcle._20). 15NdNii 5 SPMSJ20:U2
    [ 00rkoolmsamel
    [ 00Fw ]elamletopsioble
    [ 00ore

instead of

    [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd030]
    [    0.000000] Machine model: Qualcomm Technologies, Inc. APQ 8016 SBC
    [    0.000000] earlycon: msm_serial_dm0 at MMIO 0x00000000078b0000 (options '')
    [    0.000000] printk: bootconsole [msm_serial_dm0] enabled

Make sure to initialize UART_DM_DMEN correctly to fix this issue
when loading U-Boot directly after SBL (instead of through LK).

There is no functional difference when loading U-Boot through LK
since LK also initializes UART_DM_DMEN to 0x0. [1]

[1]: https://git.linaro.org/landing-teams/working/qualcomm/lk.git/tree/platform/msm_shared/uart_dm.c?h=dragonboard410c-LA.BR.1.2.7-03810-8x16.0-linaro3#n203

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>