doc: README.mxs: Add instruction to install 'libssl-dev'

Since commit bce88370

 (ARM: mxs: tools: Add mkimage support for MXS bootstream)
the following build error is seen when doing a MAKEALL build:

$ ./MAKEALL mx28evk
Configuring for mx28evk - Board: mx28evk, Options: ENV_IS_IN_MMC
mxsimage.c:18:25: fatal error: openssl/evp.h: No such file or directory

Add an entry about the need of installing the 'libssl-dev' package.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>