drivers: bus: preserve AST1700 LTPI TX driving strength

Apply ASPEED's suggested fix for the SCU_IO_PINS_TRAP_LTPI mask.

SCU_IO_PINS_TRAP_LTPI was defined as GENMASK(2, 0), which does not
match the actual LTPI-related strap bits in SCU_IO_PINS_TRAP1. Only
bit 0 (LTPI mode), bit 2 (LTPI index), and bits [15:14] (LTPI IO
driving strength) are LTPI related; the other bits covered by
GENMASK(2, 0) are unrelated straps.

This mask is used to preserve LTPI settings while clearing all other
pin/OTP straps for the remote-controller (AST1700) case, so an
incorrect mask can clear bits that should be kept or preserve bits
that should be cleared.

Tested:
Verified that the AST1700 LTPI TX driving strength remains 2
after booting into Linux.
```
root@zoro:~# devmem 0x30c02010 32
0x00008001
```
Google-Bug-Id: 568203448
Change-Id: Ia9a65cd57b5dab64012114493f0e9d73f1641cb3
Signed-off-by: Rora Yang <rora.jq.yang@fii-na.corp-partner.google.com>